The reaction of 4-hydroxy-2-alkenals and Nalpha-acetyl amino acids was studied. The product obtained from Nalpha-acetyl histidine and 4-hydroxy- 2-nonenal was shown to be a mixture of three stereoisomers. Although separation of these isomers has not been achieved, the analysis of the mixture by mass spectrometry, NMR spectroscopy, and GC/MS indicated the products are a stereoisomeric mixture of 2-N/phi or N/tau-(Nalpha- acetylhistidinyl)-4-hydroxy-2-nonenal hemiacetal derived from Michael addition of an imidazole nitrogen of N-acetylhistidine to the double bond of 4-hydroxy-2-nonenal. The elucidation of the stereochemistry of the reaction remains to be established.